The two biggest changes for me are the new way to do the muster drill and the 4 free Diamond drinks. The muster drill can be completed before even boarding the ship. Once on board, just stop by your muster station, they'll scan your Seapass and you'll be done. As odd as it may sound, I was never a big fan of the free drinks in the Diamond Lounge during Happy Hour. It was always crowded in there. With the new system, you can get your free drinks anywhere, anytime. for me, that's huge.
Another change is that you will no longer be given your Seapass at check-in. Seapasses are now waiting for you in your stateroom or on your door. If you need your Seapass for things such as beverage purchases before the rooms are ready, you show them your Set Sail Pass or give them your cabin number